Anticoagulants such as heparin exhibiting antithrombin activity, fibrinolytic agents comprising urokinase and/or tissue plasminogen activators of every kind, and antiplatelet agents have been employed for treatment of thrombosis.
Of these drugs, antiplatelet agents are made up of agents of various sorts; That is, agents preventing a reduction of a platelet cyclic AMP level, which increase a concentration of the cyclic AMP or inhibit a decomposition of the cyclic AMP, agents inhibiting a production of thromboxane A.sub.2 (TXA.sub.2) which causes platelet aggregation, agents inhibiting TXA.sub.2 activity, agents raising a level of prostaglandin I.sub.2 (PGI.sub.2) which inhibits platelet aggregation, and agents bringing on a prolongation of PGI.sub.2 activity, are involved as antiplatelet agents, and these agents had been developed and have been used as therapeutic agents.
However, of the agents stated above, agents which raise a PGI.sub.2 level or which bring on a prolongation of PGI.sub.2 activity, are very unstable, and only their analogues are being developed in form of an inclusion compound.
On the other hand, inhibitors on TXA.sub.2 activities, which are a TXA.sub.2 receptor antagonist, inhibit a production of platelet TXA.sub.2 which is produced from a phospholipid via arachidonic acid, prostaglandin G.sub.2 and prostaglandin H.sub.2, enhancing and stimulating a production of PGI.sub.2 in blood vessel walls, however, they have a weak point on their stabilities.